This report describes the facility, experimental methods, characterizations, and uncertainty analysis of the Cryo-
Distortion Measurement Facility (CDMF) at the Goddard Space Flight Center (GSFC). This facility is designed to
measure thermal distortions of structural elements as the temperature is lowered from 320K to below 40 K over multiple
cycles, and is capable of unattended running and data logging. The first measurement is the change in length and any
bending of composite tubes with Invar end-fittings. The CDMF includes a chamber that is efficiently cooled with two
cryo-coolers (one single-stage and one two-stage) rather than with liquid cryogens. Five optical ports incorporate
sapphire radiation shields - transparent to the interferometer - on each of two shrouds and a fused silica vacuum-port
window. The change in length of composite tubes is monitored continuously with displacement-measuring
interferometers; and the rotations, bending, and twisting are measured intermittently with theodolites and a surface-figure
interferometer. Nickel-coated invar mirrors and attachment mechanisms were developed and qualified by test in
the CDMF. The uncertainty in measurement of length change of 0.4 m tubes is currently estimated at 0.9 micrometers.
The diffraction efficiencies of a Fresnel zone plate (ZP), fabricated by Xradia Inc. using the electron-beam writing technique, were measured using polarized, monochromatic synchrotron radiation in the extreme ultraviolet wavelength range 3.4-22 nm. The ZP had 2 mm diameter, 3330 zones, 150 nm outer zone width, and a 1 mm central occulter. The ZP was supported by a 100 nm thick Si3N4 membrane. The diffraction patterns were recorded by CMOS imagers with phosphor coatings and with 5.2 μm or 48 μm pixels. The focused +n orders (n=1-4), the diverging -1 order, and the undiffracted 0 order were observed as functions of wavelength and off-axis tilt angle. Sub-pixel focusing of the +n orders was achieved. The measured efficiency in the +1 order was in the 5% to 30% range with the phase-shift enhanced efficiency occurring at 8.3 nm where the gold bars are partially transmitting. The +2 and higher order efficiencies were much lower than the +1 order efficiency. The efficiencies were constant when the zone plate was tilted by angles up to ±1° from the incident radiation beam. This work indicates the feasibility and benefits of using zone plates to measure the absolute EUV spectral emissions from solar and laboratory sources: relatively high EUV efficiency in the focused +1 order, good out-of-band rejection resulting from the low higher-order efficiencies and the ZP focusing properties, insensitivity to (unfocused) visible light scattered by the ZP, flat response with off-axis angle, and insensitivity to the polarization of the radiation based on the ZP circular symmetry. EUV sensors with Fresnel zone plates potentially have many advantages over existing sensors intended to accurately measure absolute EUV emission levels, such as those implemented on the GOES N-P satellites that use transmission gratings which have off-axis sensitivity variations and poor out-of-band EUV and visible light rejection, and other solar and laboratory sensors using reflection gratings which are subject to response variations caused by surface contamination and oxidation.
Two of the GOES instruments, the Imager and the Sounder, perform scans of the Earth to provide a full disc picture of the Earth. To verify the entire scan process, an image of a target that covers an 18o circular field-of-view is collimated and projected into the field of regard of each instrument. The Wide Field Collimator 2 (WFC2) 1 has many advantages over its predecessor, WFC1, including lower thermal dissipation, higher far field MTF, smaller package, and a more intuitive (faster) focusing process. The illumination source is an LED array that emits in a narrow spectral band centered at 689 nm, within the visible spectral bands of the Imager and Sounder. The illumination level can be continuously adjusted electronically. Lower thermal dissipation eliminates the need for forced convection cooling and minimizes time to reach thermal stability. The lens system has been optimized for the illumination source spectral output and athermalized to remain in focus during bulk temperature changes within the laboratory environment. The MTF of the lens is higher than that of the WFC1 at the edge of FOV. The target is focused in three orthogonal motions, controlled by an ergonomic system that saves substantial time and produces a sharper focus.
The Ultraviolet and Optical Telescope (UVOT) is one of the three
astronomical instruments onboard the SWIFT spacecraft. The optical
calibration of this instrument, which was done prior to integration
to the SWIFT spacecraft optical bench, is key to determine if UVOT
will meet its science objectives. In this paper, we describe
the optical ground support equipment (GSE) used for the
calibration of UVOT. These tests, which were carried out in the
Diffraction Grating Evaluation Facility (DGEF), at NASA Goddard
Space Flight Center, required building an optical stimulus. We
report the radiometric measurements of all the optical components
used in putting together this stimulus. This includes a vacuum
collimator with a Cassegrain design, a Pt/Cr-Ne light source, a
complete set of neutral density filters spanning 6 orders of
magnitude in transmission levels, a set of narrow-band filters
matching the center of each of the six bands of UVOT, a set of pinholes of various sizes, flat fielding diffusers, and a set of parabolic mirrors.
The GOES Imager and Sounder instruments each observe the full Earth disk, 17.4° in diameter, from geostationary orbit. Pre-launch, each instrument's dynamic scanning performance is tested using the projection of a test pattern from a wide-field collimator. We are fabricating a second wide-field collimator (WFC2) to augment this test program. The WFC2 has several significant advantages over the existing WFC1. The WFC2 target illumination system uses an array of light-emitting diodes (LEDs) radiating at 680nm, which is within the visible bands of both the Imager and Sounder. The light from the LEDs is projected through a non-Lambertian diffuser plate and the target plate to the pupil of the projection lens. The WFC2's power dissipation is much lower than that of WFC1, decreasing stabilization time and eliminating the need for cooling fans. The WFC2's custom-designed 5-element projection lens has the same effective focal length (EFL) as the WFC1 projection lens. The WFC2 lens is optimized for the LED's narrow spectral band simplifying the design and improving image quality. The target plate is mounted in a frame with a mechanized micro-positioner system that controls three degrees of freedom: tip, tilt, and focus. The tip and tilt axes intersect in the WFC's image plane, and all adjustments are controlled remotely by the operator observing the target plate through an auto-collimating telescope.